Field of the Invention
The present invention relates to a multi-chip module in which a plurality of circuit elements such as LSI chips are mounted on a high-density wiring board (also referred to as a circuit board).
Recently, multi-chip modules have become attractive in which a plurality of LSI chips are mounted on a high-density wiring board in order to speed up the operation and reduce the production cost. As is well known, multi-chip modules are classified into three types, namely a MCM-L, MCM-C and MCM-D. The multi-chip modules of the MCM-L type have a printed wiring board on which circuit elements are mounted, and enable reduction in the production cost. The multi-chip modules of the MCM-C type have a thin-film multi-layer ceramic board on which circuit elements are mounted, and enable reduction in the production cost and speeding up the operation to some extent. The multi-chip modules of the MCM-D type have a thick-film board made of ceramic or the like, on which a circuit board having at least one multi-layer structure in which an insulating layer and at least one wiring conductor layer are alternately stacked. Circuit elements are mounted on the above circuit board. As compared to the other two types, the MCM-D type multi-chip modules operate at high speeds and enable mounting of circuit elements with a high density.
FIG. 1 is a side view of a conventional MCM-D type multi-chip module, which includes a thick-film ceramic board 10 having a multi-layer structure. LSI chips 14 and passive elements 16 such as resistors and capacitors are mounted on a first surface of the thick-film ceramic board 10. These circuit elements 14 and 16 are connected to wiring lines formed inside the thick-film ceramic board 10. A large number of I/O pins 12 are attached to a second surface of the ceramic board 10 opposite to the first surface thereof. The multi-chip module is directly mounted on a printed wiring board 18 by inserting the I/O pins 12 into holes formed in the printed wiring board 18.
FIG. 2 is a side view of a conventional MCM-D type multi-chip module. In FIG. 2, parts that are the same as those shown in FIG. 1 are given the same reference numbers as previously. In order to speed up the operation and increase the density, a thin-film circuit board 20 is provided on the first surface of the thick-film ceramic board 10. The circuit elements 14 and 16 are mounted on the thin-film circuit board 20, and are connected to the thick-film ceramic board 10 and the I/O pins 12 via wiring lines of the thin-film circuit board 20. On the opposite surfaces of the boards 10 and 20, are provided pads (illustration thereof is omitted) for connections, whereby the wiring lines of the boards 10 and 20 are electrically connected together. The multi-chip module shown in FIG. 2 is directly mounted on the printed wiring board 18 by means of the I/O pins 12 in the same manner as the multi-chip module shown in FIG. 1.
FIG. 3 is a side view of a conventional MCM-D type multi-chip module. In FIG. 3, parts that are the same as those shown in FIG. 2 are given the same reference numbers as previously. The thin-film circuit board 20 is mounted on a base board 24 which does not have a wiring conductor layer. The base board 24 is made of ceramic, a silicon wafer or a metallic material such as aluminum. The base board 24 is provided in a base board mounting package 22. The package 22 is made of ceramic or mold resin, and a recess portion in which the base board 24 is accommodated. I/O pins 28 are attached to peripheral portions of the package 22 along the edges thereof. The thin-film circuit board 20 and the I/O pins 28 are connected by wires 26. Pads (not shown) for bonding the wires 26 are provided on the package 22. The multi-chip module shown in FIG. 3 is mounted on the printed wiring board 18 by means of the I/O pins 28. In the structure shown in FIG. 3, the circuit elements 14 and 16 face the printed wiring board 18.
However, the above-mentioned conventional multi-chip modules shown in FIGS. 1 through 3 have the following disadvantages.
The thick-film ceramic board 10 used in the multi-chip module shown in FIG. 1 has a wiring conductor formation density lower than that of the thin-film circuit board 20 shown in FIG. 2. Hence, a large number of stacked layers is needed to form wiring lines which realize a desired circuit configuration. Further, the larger the number of stacked layers, the longer the wiring lines. This delays transmission of signals. Hence, the structure shown in FIG. 1 is not suitable for circuit configurations particularly needed to operate at high speeds.
The multi-chip module shown in FIG. 2 utilizes the thick-film ceramic board 10 and the thin-film circuit board 10, and hence has a high production cost. The process of forming the thin-film circuit board 20 greatly depends on the surface condition (warp, roughness, pore and so on) of the ceramic part of the thick-film ceramic board 10 as well as the wiring lines (pad parts) exposed from the ceramic part. Hence, a defect will occur in the thin-film circuit board 20 and a high yield cannot be obtained if the thick-film ceramic board 10 does not have a good surface condition.
The multi-chip module shown in FIG. 3 is advantageous due to use of the base board 24 made of bulk ceramic, silicon wafer or a metallic material such as aluminum because the base board 24 can be less expensive and has a good surface condition. However, the multi-chip module shown in FIG. 3 needs the package 22 necessary to mount the multi-chip module on the printed wiring board. The use of the package 22 increases the production cost. Further, if the package 22 does not have good electrical characteristics, signals output from the multi-chip module will be delayed or contain a noise component. These phenomena degrade the performance of the multi-chip module. If there is a demand for an increased number of I/O pins 28 and/or an increased number of circuit elements mounted to the base board 24 due to a modification of the circuit configuration or the like, the package 22 needs to be redesigned. This leads to an increase in the production cost and the time necessary for redesign and reproduction.
Further, the structures shown in FIGS. 1 through 3 have a common disadvantage in that the thick-film ceramic board 10 and the base board mounting package 22 need to be redesigned each time the shape of the multi-chip module is modified. This increases the turnaround time of the multi-chip module design and production as well as the production cost.